Two carbon allotropes, carbon nanotubes (CNTs) and graphene, have attracted significant attention due to their remarkable mechanical, thermal, and electrical properties. In theory, the in-plane electrical conductivity of metallic single wall CNTs can be 1,000 times higher as compared to both silver and copper, and the tensile strength of a multiwalled CNT is expected to be in the range of 10-100 GPa. The potential applications of CNTs extend from nanoelectronics, to sensors, energy storage devices (fuel cells, batteries, and supercapacitors), photovoltaics, biomolecular imaging and detection, thermal management, and conductive nanocomposite.
Graphene is a flat monolayer of carbon atoms tightly packed into a two dimensional (2D) honeycomb lattice. Electrons in graphene behave like massless relativistic particles, which contributes to very peculiar properties such as an anomalous quantum Hall effect and the absence of localization. Graphene has demonstrated a variety of intriguing properties including high electron mobility at room temperature (15,000 cm2/V·s) and superior mechanical properties (Young's modulus is 500 GPa.). Its potential applications range from single molecule gas detection, transparent conducting electrodes and field-effect devices to energy storage devices such as supercapacitors and lithium ion batteries.
The effective utilization of CNTs and graphenes in composites and devices depends strongly on the ability to disperse them homogeneously in solvents and the matrix, and to functionalize their surfaces with target functional groups. However, it is challenging to achieve stable CNT and graphene dispersions in solvent media, as well as to functionalize their surfaces. The as-produced CNTs have a strong tendency to bundle together, and similarly graphenes tend to exist in the form of a graphite due to strong van der Waals interactions.
Surface modification of CNT and graphene with small molecules or polymers is one way to attempt to increase their solubility and provide desired functionalities. The surface modification generally involves attaching functional groups to CNT/graphene surfaces through the formation of covalent bonds (covalent approaches) or non-covalent bonds (non-covalent approaches). Although the covalent approach is generally effective in functionalizing CNTs and graphenes, the covalent bonding disrupts the long range π conjugation of the CNT, leading to degraded electrical properties and diminished mechanical strength.
In contrast, non-covalent approaches can utilize multiple weak interactions such as π-π interactions, van der Waals interactions, and static charge interactions. Such non-covalent interactions avoid damage to the chemical structure, and allows the CNT or graphene to retain their electrical and mechanical properties. However, although significant effort has been invested in this pursuit, the commercial application of CNTs and graphene is still extremely limited, mainly due to the lack of a simple and versatile system to disperse and functionalize CNTs and graphenes.